FAQ on Genetics

1. I know that we inherit most of our mental, physical and biological characteristics from our parents. Apparently these characteristics are passed on to us through our genes. What is a gene?

 Our human organism consists of and functions through 35000 or more different kinds of proteins. These proteins are manufactured as per a series of coded instructions, called genes. These genes (coded instructions) are stored in molecules known as DNA, in the nucleus of every human cell.

2. I have heard of chromosomes in the human cell; X-chromosome and Y-chromosome, determining the sex of the child to be born. But what is this DNA?

 Yes. There are 46 chromosomes in every human cell, 23 numbers inherited from each parent. Each chromosome contains one DNA molecule in addition to other proteins. DNA expands as Dioxy-ribo Nucleic Acid. Each chromosome is a package for one very long continuous strand of the DNA, wound around cores of protein known as histone. Large proportion of DNA is composed of repeated sequences of codes, but all may not encode proteins. Only a few sections of the DNA code sequence, estimated as only 5% of human DNA chain, encode proteins. These encoded sections of DNA are called Genes. About 35000 genes are spread over the 46 DNA chains packaged within the 46 chromosomes in the human cell. These genes are only a collection of plans for manufacturing proteins, one gene for each protein.

 3. What are proteins and what part they play in development and sustenance of human body?

 First let us consider the Proteins which are in the nucleus and elsewhere. The proteins act as enzymes and other structural components of the living cell. A complex organism such as a human has about 35000 different kinds of protein molecules in their body. Each protein is good for a different function. They are the major work horses of an organism. They do virtually all the important jobs we associate with life.

  • They act as enzymes to catalyse and control all the chemical reactions in an organism.
  • They form prominent structures – hair, skin, cartilage, bones etc
  • They form tissues and filaments to act on our muscles
  • They constitute harmones to communicate between different kinds of cells
  • They form receptors for, say,  tasting and smelling
  • They filter and transport small molecules and ions across cell membranes
  • They act as regulatory elements to control our metabolic rate.

These proteins consist of long chains of 20 different amino acids as polypeptide chains.  The primary structure basically determines the properties of every protein, including the way the long chain of amino acids folds up rigidly into specific shape. This is mainly due to attractions between the atoms of each of the 300 and odd constituent molecules in the polypeptide chain. Many functional proteins consist of a tight cluster of several polypeptides with a shape and form appropriate to its function.

4.  Are there different kinds of cells in our body? Yes, naturally it must be so.

 The cells are not identical even in a single organism. They are different depending on their functions. However the cell structure remains the same. It has a nucleus in the centre covered by an envelope and then surrounded by cytoplasm and many other materials for processing and storing nutrients from food molecules. The whole cell is covered by a thin membrane that keeps the cell intact and creates entries and exits. This membrane decides what can get in (like nutrients) and get out (like salt and sugar), depending on their functions.

 5. How do these cells become different and unique?

 A fertilized human egg cell is called a zygote. A zygote develops from a single cell – into an embryo, a complex of many specialized cells. A zygote is toti-potent. Through successive growth, replications and divisions, it has the potential to become any kind of specialised cell such as skin or brain cell. At some stage during this development, each cell is determined to become a particular specialized cell. These specialized cells lose their toti-potency and produce only similar specialised cells by the process of subsequent cell divisions.

 6. Do all genes produce proteins?

 Genes do not produce proteins; they are produced by organisms that regulate gene expressions by turning them on and off as needed. A regulatory gene produces such inhibitors. Most living things are composed of different kinds of cells to perform different functions. A liver cell and a nerve cell have different biochemical duties to perform. But every cell of our organism has the same set of DNAs lodged in the 46 chromosomes of their nucleus, one DNA corresponding to each chromosome. Specific genes are turned on as required in every different type of cells. Such timely cell-specific gene expressions are more in evidence in the development of a fertilized egg into an embryo and then into a growing human form. There are some ‘master genes’ which control such body plans.

 7. What are Stem Cells?

 As mentioned earlier a zygote is toti-potent, that is, through successive replication and division it has the potential to become any kind of specialised cell such as skin or brain cell. These specialized cells lose their toti-potency at some stage and produce only similar specialised cells by the process of cell divisions. No amount of experimental tweaking can restore their toti-potency. However some cells may retain their toti-potency and can be tweaked to act as such. There are also cells which are termed as pluri-potent and multi potent. These cells can develop into some of the specialised cells. These cells can by themselves repair any loss or damage to their respective specialised cells. Toti-potent, pluri-potent and multi-potent cells are commonly called as stem cells. The developed cell of a mammary gland can provide the toti-potent nucleus required for the cloning process. These stem cells are useful for various kinds of therapeutic treatments in humans. Obtaining these stem cells is difficult. Early human embryos are a ready source for stem cells which are toti-potent. The umbilical chord, testes, mammary glands, bone-marrow are some of the parts where we may get stem cells, though not toti-potent.    

 8. What are Germ Cells?

Germ cells are sex cells, stored in ovaries and testes (genital organs). Though these special cells are formed at the time of birth, they are activated only at the time of puberty. There exists a different cell division process for these Germ Cells. It is called meosis, and it halves the chromosomes in these cells (gametes). During the formation of zygote by sperm fertilising the egg, the chromosomes number is again restored by adding the two halves. As the zygote grows most parts of the foetus are made of somatic cells (with the full set of chromosomes) formed by the process of mitosis. Gametes are derived by the process of meiosis from special germ cells (with only half set of chromosomes) that reside only in the ovaries and testes. They become sperms and eggs in males and females, respectively.

9. What is cloning?

Cloning of an organism is a process that deals with the manipulation of a fertilized egg. The nucleus of a fertilized egg contains a mixture of 23 chromosomes each from father and mother. This nucleus is removed and replaced by a nucleus from a somatic cell of a donor. This donor nucleus contains all the 46 chromosomes of the donor. If this manipulated zygote is allowed to grow, it will be genetically identical to the donor. The formation of identical twins, triplets, etc is a kind of natural cloning in humans. Identical siblings result from accidental premature division of the zygote to make two or more cells (zygotes) with identical set of chromosomes containing the same genes.

The famous Dolly, the sheep that was a product of cloning, had in fact three mothers. A cell was removed from one sheep from her mammary gland. The same was allowed to divide. At a particular stage in cell cycle it was inserted into a fertilized egg removed from another sheep, after removing the naturally formed nucleus from the zygote. The inserted donor cell acted as the new nucleus with same set chromosomes as the donor. The zygote then was placed in the womb of a third sheep to act as a surrogate mother. After several trials Dolly was delivered successfully and was a cloned version of the first sheep.

10. What are the controversies in Gene Therapy?

All scientific advancement has an up side and a down side. It can be used for peaceful and developmental purposes, or, when in wrong hands, it can be used for unscrupulous purposes and  in a way harmful to the human society in general. The genetic science and engineering have much more far-reaching up and down sides. There are three areas of genetic research causing concern about their misuses.

a) Somatic gene therapy: This deals with genes of a particular individual, and will not be passed on to the next generation. It is more like a medicine, improving the heath of the particular individual, by removing the error in the genetic code, which causes wrong type of protein to be produced, causing decease. Recently several such deceases have been found to be due to such errors and they have been cured. Though there are other methods for correcting such errors in gene code, stem cells offer a great help in finding such remedies. Stem cell research often needs toti-potent cells as available in early embryos. This leads to illegal harvesting of such embryos raising severe ethical issues.

b) Germ-line gene therapy: The gene therapy is considered as the greatest advantage of genetic research. To ensure the defective genes found in somatic cells, do not get passed on to the next generation even by chance, the germ cells need to be altered. This germ-line research is fraught with several grave consequences including large scale misuse. Apart from ethical aspects of unwarranted interference with the natural process of conception and delivery, the misuse may include: attempts to make designer babies, erred procedure leading to production of individuals with potentially dangerous abnormalities, risk of introducing an abnormality into several future generations, etc. More knowledge and more safeguards are needed before we can allow such therapies.

c) Cloning: Nature has built-in enormous diversity universally among all organisms to obtain a fine ecological balance. In this respect cloning does not offer any significant advantage. It provides some limited facilities for research in gene therapy. Otherwise cloning procedure has the same objections regarding the use of human eggs and zygotes and when misused or erred, could result in same hazards as germ-line therapy.

More we learn about DNAs and genetics of a complex organism like humans, more unknown things are coming to the fore; like we carry 95% of DNAs which do not produce proteins. It is not clear what their functions in the organisms are. A genetic code is known to produce one kind of protein at a point of time, and some other protein when read again later. The functions of many kinds of proteins are not clearly understood. Though partial success has been achieved in mapping the entire genome (complete code of all DNAs) of a human, we are a long way from understanding the full significance of the same. “Geneticist will have high ideals for the application of their research. In practice, power to apply that knowledge will rest with others.”    

References:

1. Genetics – A beginner’s guide : B.Guttman, A.Griffiths, D.Suzuki, T.Cullis – (2002)

2. Cracking the genetic code of life – DVD produced by Elizabeth Arledge & Julia Cort for  NOVA (2004)

3. DNA from the beginning – An internet book by Dolan DNA Learning Center, Cold Spring  Harbour Laboratory – (2002)

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